1. Field of the Invention
The present invention relates to an image quality stabilization method executed by an image forming apparatus.
2. Description of the Related Art
Recently, an electrophotographic image forming apparatus and an inkjet image forming apparatus have been widely used. It is desired by the market that an image forming apparatus of this type be capable of printing an image at a sufficiently high quality.
As one of the causes of image degradation, density unevenness, which may occur in the sheet conveyance direction (sub scanning direction), may occur during image forming. In order to prevent density unevenness in the sub scanning direction, Japanese Patent Application Laid-Open No. 2007-108246 discusses the following method.
More specifically, the method discussed in Japanese Patent Application Laid-Open No. 2007-108246 previously measures the level of density unevenness in the sub scanning direction that occurs according to the cycle of an outer periphery of a photosensitive member (photosensitive drum) in association with the phase of the photosensitive member. Furthermore, the method discussed in Japanese Patent Application Laid-Open No. 2007-108246 stores the result of the measurement on a storage unit in a density pattern information table.
Then, information about density unevenness, which has been associated with the phase of the photosensitive member during image forming, is read from the density pattern information table. If a high density is acquired, the method discussed in Japanese Patent Application Laid-Open No. 2007-108246 lowers the image forming density. In other words, Japanese Patent Application Laid-Open No. 2007-108246 discusses reverse density correction control.
To describe the above-described phenomenon of density unevenness in more detail, the variation of a rotation speed of the photosensitive member may be one of the specific causes of density unevenness in the sub scanning direction. To paraphrase this, if the rotation speed of the photosensitive member is low, the position of an electrostatic latent image formed on the photosensitive member may shift in the rotational direction of the photosensitive member (in the upstream direction of the image). Accordingly, the interval between static image lines may decrease.
On the other hand, if the rotation speed of the photosensitive member is high, the electrostatic latent image is displaced in a direction reverse to the rotational direction of the photosensitive member (in the downstream direction of the image). Accordingly, in this case, the interval between static image lines may increase.
Furthermore, if the rotation speed of the photosensitive member is low when the toner adhered to the electrostatic latent image forming position is primarily transferred from the photosensitive member to an intermediate transfer member, then the position of the image after the primary transfer shifts in a reverse direction of the rotational direction of the photosensitive member (i.e., in the downstream direction of the image). Accordingly, in this case, the interval between static image lines may increase.
On the other hand, if the rotation speed of the photosensitive member is high, the position of the image after the transfer shifts in the rotation direction of the photosensitive member (in the upstream direction of the image). Accordingly, the interval between line images may decrease.
As described above, an electrostatic latent image and an image may be displaced due to the variation of the rotation speed of the photosensitive member. Accordingly, the density of the image formed on the intermediate transfer member may become uneven. To macroscopically observe an image having an uneven density, the density of a region in which the image has been highly densely formed may appear to be high. On the other hand, the density of another region in which the image has been loosely formed may appear to be low. As a result, an observer of the image may recognize that density unevenness has occurred on the image.
As described above, in order to primarily prevent density unevenness, Japanese Patent Application Laid-Open No. 2007-108246 discusses a method for correcting the uneven density by reverse density correction.
However, the density correction method cannot solve the very problem of uneven intervals between line images. To paraphrase this, it is desired by the market to introduce a method for more directly solving the problem of unevenly formed line images.